The invention relates to a method of regulating the air-fuel mixture in the case of an internal-combustion engine of a motor vehicle in a closed control loop, where a lambda setpoint is transferred to a controller for influencing an injection calculation for the internal-combustion engine. The actual lambda value, which occurs at the output of a controlled system as a function of the injection calculation, is returned to the controller.
The reduction of exhaust emissions represents a central theme in the development of modern motor vehicles. For reaching certain target values and/or for observing legally prescribed limit values for exhaust emissions, very high technical expenditures are required.
According to the state of the art, three-way catalysts are frequently used for the reduction of exhaust emissions in Otto-engine-related combustion. The three-way catalyst has its maximal conversion rate in a narrow lambda window about the stoichiometric air/fuel ratio (that is, lambda=1). A module in the engine control unit takes over the controlling and regulating of the lambda value to the optimal desired value. The entire module for the lambda control is typically constructed of several submodules. Thus, for example, dynamic effects occurring in addition to the pilot control and regulating of the lambda value are compensated, such as the build-up and reduction of the wall film. Particularly when the storage capacity of oxygen of the catalyst is reduced due to aging, a fast settling to the desired value is important for minimizing the exhaust emissions. The pilot control and other correction measures alone are not sufficient for optimally guiding the lambda value in the transient operation. The lambda control is therefore one of the most important control loops in the transmission line.
The controlled system G(s) relevant to the lambda control can be approximated by a delay element of the first order with dead time. The following can be formulated as the transfer function of the controlled system:
      G    ⁡          (      s      )        =            1              1        +                              T            ⁡                          (                                                r                  L                                ,                                  n                  eng                                            )                                ⁢          s                      ·          ⅇ                        -                                    T              1                        ⁡                          (                                                r                  L                                ,                                  n                  eng                                            )                                      ⁢        s            
This is a non-linear system with dynamics depending on the operating point (relative air filling rL, rotational engine speed neng) and dominant dead time Tt. The time constant T is characterized by the response characteristic of the broadband lambda probe (diffusion time of the oxygen molecules). The dead time is mainly but not exclusively, a function of the position of the probe in the exhaust line.
The time constants Tt and T of the controlled system may change, among other things, as a result of the aging of the broadband probe and the engine model variation. The concepts of the lambda control known from the state of the art—these are usually robust controllers (such as H∞ controllers) designed offline in the frequency domain—, however, cannot take such a change into account. Thus, it cannot be ensured that the control is optimally adapted to the real controlled system under all circumstances.
In the case of most methods known from the state of the art, the parameters of the controller have to be stored in the electronic control unit as operating-point-dependent characteristic maps. They therefore occupy a very large amount of application data memory there. As a result of the control algorithm to be calculated at high expenditures, additionally much computing time is used in the electronic control unit only for the lambda control. Changes in the dimensioning of the exhaust system of an engine or motor vehicle have a direct effect on the parameters of the lambda control; that is, the determination of the control parameters has to be carried out again. Because of the high-expenditure calculation of the controller parameters, an adaptation of the parameters in the electronic control unit during the operation of the control loop is not possible. In order to ensure the stability of a system with a pronounced dead time characteristic, the control has to be designed very conservatively, which has the result that often a relatively large amount of dynamics are “given away” in the control loop characteristics. This means that, because of the design of the control, the control loop normally reacts very slowly.
It is an object of the invention, to provide a method of the above-mentioned type by which control is better coordinated with the controlled system.
According to the invention, at least one system parameter of the controlled system is determined and the determined system parameter is transferred as a parameter to a Smith predictor, which is added to the controller for compensating the influence of the system dead time on the control loop characteristic.
Preferably, the system dead time is determined and transferred as a system parameter of the controlled system. The dominating influence of the system dead time can thereby be compensated by the use of a Smith predictor. Desired system characteristics can therefore be adjusted according to the usual demands on the lambda control (emissions, movability, catalyst window).
The exhaust emissions achievable by a method according to the invention are below (or not more than on the same order of) the exhaust gas emissions of a control according to the state of the art. However, it is a significant advantage of the invention that, when the invention is applied, the consumption of resources in the electronic control unit can clearly be reduced in comparison to the state of the art.
Preferably, at least one parameter of the Smith predictor, particularly the parameter concerning the transferred system parameter (this is preferably the system dead time) can be changed online, that is, during the operation of the control loop. This advantageous further development of the invention makes it possible to optimally coordinate the control with a change of the system parameters. The system dead time or another system parameter can then be newly determined during the operating time and can be transferred to the Smith predictor. The new determination and transfer can, for example, take place continuously, quasi-continuously, at regular intervals, or in an event-controlled manner. The lambda control according to such a further development of the invention is therefore suitable to adapt itself in a self-learning manner to changed system parameters.
At least one system parameter of the controlled system, particularly the system dead time, is preferably determined by an analysis of the variation in time of the actual lambda value as a result of a forced excitation fed into the control loop. The forced excitation can particularly be modulated upon the lambda setpoint.
As known from the state of the art, the forced excitation can be calculated out of the actual lambda signal again by way of a lambda model in order not to excite the control.
In particular, the system dead time can be determined in that the time shift is determined between a signal edge of the forced excitation and a resulting change of the actual lambda value. When determining the dead time, prior knowledge is preferably utilized with respect to an expected value of the dead time. The prior knowledge may exist, for example, in the form of a range of plausible values for the system dead time. Also when determining other system parameters, such as a time constant of a PT1 member, as required, prior knowledge with respect to the system parameter to be identified may be advantageously utilized.
In particular, the system dead time concerning curve fitting algorithms or regression calculation can be computed. The determined value of the system dead time can be returned into the Smith predictor of the control and cause a model tracking there.
As another synergistic effect, the estimated or otherwise determined dead time can also be used in a lambda model. As described above, such a lambda model can be used for calculating a forced excitation out of the actual lambda signal again; that is, for generating a corrected actual lambda signal from an uncorrected actual lambda signal. Thus, in addition to the system model of the controller, the lambda model can also be tracked. In addition, the tracked lambda model can also be used for calculating the load signal by way of the injection and therefore cause the measurement by way of the hot-film air mass meter (HFM) to be eliminated.
The forced excitation is, preferably, not exclusively used for the parameter identification, particularly the identification of the system dead time, but additionally for the catalyst and lambda probe diagnosis. If a forced excitation is provided for such purposes anyhow, no additional excitation will be required. No other interference therefore has to be introduced into the system.
As required, a forced excitation existing anyhow can be modified for the purpose of parameter identification in such a manner that it continues to achieve its original purpose.
A parameter identification of at least one system parameter by analyzing a forced excitation can, in principle, also be used in the case of other model-based methods of the above-mentioned type, that is, methods that are not based on the use of a Smith predictor, and also at least partially have the above-mentioned advantages.
In principle, a system parameter identified in such a manner can also be used exclusively for the tracking of the lambda model.
Likewise, an adaptation of the parameters of a system model used in a model-based method corresponding to the present description can basically also be used in the case of other model-based methods of the above-mentioned type, that is, methods that are not based on the use of a Smith predictor, and then also at least partially has the mentioned advantages.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.